DSO & MSO (Block Diagram)

DIGITAL STORAGE OSCILLOSCOPE (DSO)

The digital storage oscilloscope eliminates the disadvantages of the analog storage oscilloscope. It replaces the unreliable storage method used in analog storage scopes with digital storage with the help of memory. The memory can store data as long as required without degradation. It also allows the complex processing of the signal by the high-speed digital signal processing circuits. 

In this digital storage oscilloscope, the waveform to be stored is digitised and then stored in digital memory. The conventional cathode ray tube is used in this oscilloscope hence the cost is less. The power to be applied to memory is small and can be supplied by small battery. Due to this the stored image can be displayed indefinitely as long as power is supplied to memory. Once the waveform is digitised then it can be further loaded into the computer and can be analysed in detail.

Block Diagram

As done in all the oscilloscopes, the input signal is applied to the amplifier and attenuator section. The oscilloscope uses same type of amplifier and attenuator circuitry as used in conventional oscilloscopes. The attenuated signal is then applied to the vertical amplifier.

The vertical input, after passing through the vertical amplifier, is digitised by an analog to digital converter to create a data set that is stored in the memory. The data set is processed by the microprocessor and then sent to the display.

To digitse the analog signal, analog to digital (A/ D) converter is used. The output of the veräcal amplifier is applied to the A/D converter section. The main requirement of A/D converter in the digital storage oscilloscope is its speed, while in digital voltmeters accuracy and resolution were the main requirements. The digitised output is needed only in the binary form and not in BCD. The successive approximation type of A/D converter is most often used in digital storage oscilloscopes.

Digitising the analog input signal means taking samples at periodic intervals of the input signal. The rate of sampling should be at least twice as fast as the highest frequency present in the input signal, according to the sampling theorem. This ensures no loss of information. Sampling rates as high as 100,000 samples per second is used. This requires very fast conversion rate of A/D converter.

If a 12-bit converter is used, 0.025 % resolution is obtained while if 10-bit A/D converter is used then resolution of 0.1 % (1 part in 1024) is obtained. Similarly, with 10-bit A/D converter, the frequency response of 25 kHz is obtained. The total digital memory storage capacity is 4096 for a single channel, 2048 for two channels each and 1024 for four channels each.

The sampling rate and memory size are selected depending upon the duration and the waveform to be recorded. Once the input signal is sampled, the A/D converter digitises it. The signal is then captured in the memory. Once it is stored in the memory, many manipulations are possible as memory can be read out without being erased.

Acquisition Methods

In the digital storage oscilloscope, it is necessary to capture the digital signal and store it. Depending upon the particular application, there are three different acquisition methods used in the digital storage oscilloscopes. These three methods are :

1. Real-time sampling.

2. Random repetitive sampling.

3. Sequential repetitive sampling.

Real-Time Sampling

This is the most straightforward method of digital signal capturing. In this method, in response to a single trigger event, the complete record of nm samples is simultaneously captured on each and every channel. From these samples recorded in a single acquisition cycle, the waveform is displayed on the screen of a digital storage oscilloscope.

Three important features of this method are,

1. The display and analysis of the waveform can be carried  out at a later stage while the signal gets recorded in memory at an earlier stage

2. It is very easy to capture the signals that happen before 'the trigger event.

3. A truly simultaneous capture of multiple Signals is automatic

This method can be used in a continuously repeating mode but each waveform displayed is captured from a single acquistion cycle. The larger memory and fast sampling rate plays an important role in the real time sampling. The higher sampling rate is required to capture long time interval signal capturing. This is possible due to large memory. The signal fluctuations occurring entirely between samples will not be captured in the sample record.

The sampling theorem helps to select the proper sampling rate. It states that if a Signal is sampled greater than twice the frequency of highest frequency component in signal then the original signal can be reconstructed, exactly from the samples. Half the sampling frequency is called Nyquist limit or Nyquist critical frequency. This is denoted as fc.

Any signal component having a frequency higher than fc gets falsely translated to another frequency somewhere between d.c. and fc by the act of sampling. This is called aliasing. A signal of frequency fc+A will be aliased to fc—A for fc. The 3 dB bandwidth of the vertical amplifier should be less than fc at the fastest sampling rate. In practice the 3 dB bandwidth is set to fs / 4 with about 5 % overshoot where fs is the sampling frequency.

Random Repetitive Sampling

The bandwidth is limited to fs / 4 in real-time sampling. The major disadvantage of this is increasing bandwidth means increasing sampling rate and fast sample rate digitizers and memory are very expensive. The method in which the bandwidth is not limited by sampling repetitive method.

In this method, repeated real-time data acquisition cycles are performed. Still, each sample value is plotted independently on display as a dot. Interpolation between samples is not done. Each acquisition cycle produces a random time interval td between the trigger point and the sample clock as shown in Fig. 

The time between the samples from that capture is ts with an offset of td from the trigger point. The trigger interpolator measures the time interval td on each acquisition cycle. It is located in the time base.

Each successive acquisition is plotted at its measured random offset. This progressively fills the picture of the waveform. As the waveform fills in, the gaps between the dots become smaller and the effective sampling rate increases. Accuracy of the trigger interpolater while measuring td limits the effective sampling rate. The disadvantage of this method is that the ability to capture a nonrecurring transient is lost.

Sequential Repetitive Sampling

An oscilloscope having bandwidth 20 to 50 GHz need very fast sweep speed settings. In such cases, the random repetitive method can not work satisfactorily. Hence sequential repetitive sampling is used.

In this method, one sample value per trigger event is captured at a carefully controlled time delay fds after the triggering pulse, as shown in Fig. 

This delay is increased by a small amount tse after each point is captured. The single sample acquisition cycle is repeated till the entire waveform has been plotted. In this method the increase in delay which is tde is the effecåve sample Hme.

This method can not capture trigger events or any pretrigger information. The major disadvantage of this method is pretrigger view feature gets lost. Hence this method is used only in microwave bandwidth digital oscilloscopes.

Special Functions

The digital storage oscilloscope has a variety of special functions. These special functions are,

1. Pretrigger View: The oscilloscope has a special feature called pretrigger view. This mode means that the oscilloscope can display what has happened before a trigger input is applied. This selection is a percentage selection. This mode of operation is useful when a failure occurs. The pretrigger can be 25 %, 50 %,75 % for the single shot mode.

2. Channel Difference (A - B): This depends on the reference cursor state. When the reference cursor is active, the function calculates the difference between channel A and channel B, where the reference cursor level is considered to be zero. When the reference cursor is inactive, the function calculates the difference between channel A and channel B, where the 0 V level is considered to be zero.

3. Channel Add (A + B): Similar to the difference, this function calculates the addition of channel A and the channel B, depending on the reference cursor state.

4. Channel A Inversion (-A): This calculates the inversion of channel A.

5. Channel B Inversion (-B): This calculates the inversion of channel B.

6. X-Y Function: The X-Y function window is opened after activation of the X-Y function. This displays the data visible on the main screen. The zoom function affects the amount of displayed data. The 0 V values for X-axis and Y-axis are displayed and can be changed by vertical shift for both axes.

7. Glitch Detect: The digital storage oscilloscope can capture a glitch up to 100 ns width and also can capture positive, negative or alternate positive and negative glitches.

Trigger Coupling: By selecting a proper coupling, the trigger signal can be

processed before applying to the comparator.

i) D.C. coupling: Useful for low-frequency signals.

ii) A.C. coupling: Useful for low-level signals having a d.c. component.

iii) H.F. reject: Useful for rejecting high-frequency noise while viewing low-frequency signals.

iv) L.F. reject: Useful for rejecting low-frequency signals such as jitter due to low-frequency noise.

Automatic Measurements

In the digital oscilloscope, the waveforms can be easily made available to the computer and due to the involvement of the computer the digital oscilloscope can have a variety of useful features. Automatic measurements is one of such powerful features of a digital oscilloscope. Some of the uses of automatic measurements are,

1. Calibraåon

2. Autoscale

3. Measurement of parameters

4. Mathematical operations.

Advantages of D.S.O.

• It is easier to operate and has more capability.
• The storage time is infinite.
• The display flexibility is available. The number of traces that can be stored and recalled depends on the size of the memory.
• The cursor measurement is possible.
• The characters can be displayed on the screen along with the waveform which can indicate waveform information such as minimum, maximum, frequency, amplitude etc.
• The X-Y plots, B-H curves, and P-V diagrams can be displayed.
• The pretrigger viewing feature allows displaying the waveform before trigger pulse.
• Keeping the records is possible by transmitting the data to a computer system where further processing is possible
• Signal processing is possible which includes translating the raw data into finished information e.g. computing parameters of a captured signal like r.m.s. value, energy stored etc.
• Brighter and bigger display with colour to distinguish multiple traces.
• Equivalent Eme sampling and average cross-consecutive samples lead to higher resolution down to gV.
• Slow traces like the temperature variation across a day can be recorded.
• The digital technique allows quantitative analysis.
• The memory can be arranged not only as a one-dimensional list but also as a two-dimensional array.
• The built-in interfaces such as RS 232 serial port, centronix parallel, IEEE 488 Bus are available.

Applications of D.S.O.

• Measurement of various a.c. and d.c. parameters such as currents, vo tages etc.
• Measurement of various parameters of alternating signal such as r.m.s., average, crest factor, duty cycle etc.
• Measurement of frequency, time period, phase, and phase difference for periodic and nonperiodic waveforms.
• The transient parameters of fast changing waveforms such as overshoot, rise time, fall time etc. can be measured.
• Mathematical operations such as addition, subtraction, integration etc. of various waveforms can be obtained.
• Used to measure slow moving parameters such as temperature of the day.
• The operations such as fast Fourier transform, discrete Fourier transform, inverse Fourier transform etc. can be performed.
• The parameters like inductance, capacitance, impedance etc. also can be measured.
• For component testing and troubleshooting as the transients can be captured and stored.
• For transmission line analysis to obtain standing waves, modulation characteristics etc.
• The visual representation of a target for an aeroplane, ship etc. can be obtained.
• The characteristics of various components such as V-1 characteristics of diodes, transistors etc. can be obtained.
• To obtain the P-V diagrams, B-H curves, Hysteresis loops etc.